Opinion polls continue to show the gap between a resurgent SNP and Labour growing, not narrowing. But perhaps most remarkably, to the obvious bemusement of the prime minister on his sojourns north to warn Scots of a Zimbabwean-style economic meltdown post-independence, that most conservative of institutions - the Scottish business community - is becoming a key catalyst for change. Barely a week goes by without another spectacular business coup for the nationalists - Kwik-Fit tycoon Sir Tom Farmer, Stagecoach boss Brian Souter, former RBS chairman Sir George Mathewson and many other apparently unlikely SNP supporters.

It's an astonishing turnaround compared with 1999, when 100 Scottish businessmen and women signed a full-page Labour advertisement warning against the impending disaster of an SNP government.

So how has this happened? The sheer political ability of Alex Salmond should not be underestimated. But most, if not all, of the independence converts will also have listened to an impressive pitch on the economics of independence. Developed with evangelical zeal by the party's enterprise spokesman, Jim Mather, the SNP's economics presentation sets out the key message that, confined within the union, Scotland is being denied the powers to compete in the global marketplace.

Careful research into the extraordinary success of western Europe's small open economies informs much of the party's current thinking. Deep cuts in corporation tax, the abolition or merger of named government quangos, a policy of low business rates, and a big commitment to pre-school and early-years education are key planks of SNP policy.

In response to all this, Labour has employed its brightest brain (and Brown protege) - Wendy Alexander, sister of the transport secretary, Douglas Alexander - to trash the nationalists' figures. Bizarrely, this has meant that it is now Labour, and not the SNP, that talks most about North Sea oil.

In a passage of his budget speech little noticed in England, the ever political chancellor conveniently downgraded his forecasts for future oil revenues. In a choreographed move, Wendy Alexander then claimed that an independent Scotland would face a £12.9bn deficit on independence. Labour now puts the independence bill at £5,000 for every family unlucky enough to live north of the border.

The SNP's reaction has been instructive. Relying on the collapse of trust in New Labour, rather than engaging in an argument over specific figures, the SNP says Labour is making up the numbers as it goes along. And anyway, add the Nats, we can talk about all of this when we call the referendum on independence in three years.

In truth, Labour's claims of an astronomical deficit seem an odd election strategy. In England the chancellor trumpets his record of producing stable public finances. In Scotland the Labour message appears to be: "Vote for us, we've run up the biggest per-capita deficit in Europe."

This is not the only economic inconsistency when it comes to Scotland. For independence supporters it has always been absurd to watch the chancellor lecturing his European counterparts on the absolute necessity of tax competition between states, only to return home to Scotland to argue just as forcefully for his own country to be excluded from that apparently vital competition.

It is an absurdity recognised by more and more Scottish businesses, and on May 3 it could lead to the end of 50 years of Labour hegemony in Scotland.

· Ewan Crawford was private secretary to John Swinney, the former leader of the Scottish National party